1. Field of the Invention
The present invention relates to a modularized active power filter and, specifically, to a modularized active power filter including a control module and at least one power module. The control module automatically detects the number of power modules and generates one set of control signals corresponding to the number of power modules thereof, so as to provide a final compensation capacity generated by the modularized active power filter to compensate for harmonic currents and reactive power of a load.
2. Description of the Related Art
Recently, the characteristics of power electronic devices have improved significantly. The power electronic devices with characteristics of high-voltage rating, high-current rating, and high switching speed have been developed due to the improvement of semiconductor manufacturing technique. Power electronic devices are widely applied in electric power equipment, such as an uninterruptible power supply, motor driver, arc furnace, trolley car, battery charger, lighting appliance, etc. The electric power equipment may generate a large amount of harmonic currents due to the nonlinear input characteristic of such loads. The harmonic currents will pollute the distribution power system resulting in serious problems such as transformer overheat, rotary machine vibration, degrading voltage quality, electric power components destruction, medical facilities malfunction, etc. In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC1000-3-5, IEC1000-3-4, etc., have been established by international research centers.
Recently, active power filters are developed for suppressing harmonic currents and compensating for the reactive power. Referring initially to FIG. 1, a schematic circuitry of a conventional active power filter is illustrated. A power source 7 provides a load 6 with an electrical power through a power line 10, and an active power filter 8 connects in parallel with the load 6 for providing a compensating current. Moreover, the compensating current equals a summation of the fundamental reactive current and the harmonic currents of the load 6. Subsequently, the compensating current injects into the power line 10 between the power source 7 and the load 6 to suppress the harmonic currents of the load 6 and improve the power factor simultaneously. Both conventional active power filters disclosed in U.S. Pat. Nos. 5,677,832 and 5,614,770 can be categorized as the same kind of described active power filter 8. However, the active power filter 8 consists of a control circuit and a power circuit in one single set, which causes the manufacturer much inconvenience in assembly, test, mass production, and service of the active power filter 8. On the other hand, users of the conventional active power filters have to replace the active power filter 8 with another one which has a larger compensation capacity while the compensation demand of the load 6 increases. This will cause a waste in finance. Although some of the conventional active power filters have a function of parallel-operation, a plurality of active power filters can be connected in parallel to increase the compensation capacity when the compensation demand of the load 6 increases. However, the manufacturing cost cannot be decreased substantially since each active power filter still requires a control circuit and a power circuit.
For increasing flexibility in an active power filter application, an active power filter 9 with prior modularized structure was developed, which is shown in FIG. 2. The active power filter 9 comprises a control module 91 and a power module 92. The control module 91 has a primary control circuit 911 and a current sensing circuit 912. The power module 92 has a power inverter 921, a voltage/current sensing circuit 922, an auxiliary control circuit 923, a pulse width modulation (PWM) circuit 924, and a driving circuit 925. The power inverter 921 provides an AC output terminal 9211 and a DC output terminal 9212. The AC output terminal 9211 connects to the power line 10. When the conventional modularized active power filter 9 is used to filter out the harmonic currents and compensate for the reactive power of the load 6, the current sensing circuit 912 of the control module 91 detects the current of the load 6 and transmits a signal to the primary control circuit 911. Thereby, the primary control circuit 911 generates a current control signal for the auxiliary control circuit 923 of the power module 92. Meanwhile, the voltage/current sensing circuit 922 of the power module 92 detects a compensating current of the inverter 921 and a voltage of the DC output terminal 9212 and, then, sends a compensating current signal and a voltage signal to the auxiliary control circuit 923.
According to the current control signal generated by the primary control circuit 911 and the voltage signal as well as the compensating current signal detected by the voltage/current sensing current 922, the auxiliary control circuit 923 generates a modulation signal and sends it to the PWM circuit 924 for generating a PWM signal. The PWM signals generated by the PWM circuit 924 are sent to the driving circuit 925, so as to generate a plurality of drive signals for controlling a plurality of power electronic switches of the inverter 921 to generate a compensating current. The compensating current equals a summation of the fundamental reactive current and the harmonic currents of the load 6 and is injected into the power line 10 for suppressing the harmonic currents of the load 6 and improving the power factor.
Still referring to FIG. 2, the compensation capacity of the conventional modularized active power filter 9 can be enlarged flexibly by increasing the number of power modules 92 while the compensation demand of the load 6 increases. In comparison with the conventional active power filter 8 shown in FIG. 1, the conventional modularized active power filter 9 provides a better flexibility of compensation capacity. Nevertheless, for completing the close loop control to generate the PWM signal and then control the inverter 921 through the driving circuit 925, the auxiliary control circuit 923 is still necessary in the power module 92 to receive the current control signal from the primary control circuit 911 of the control module 91, the compensating current signal and the voltage signal from the voltage/current sensing circuit 922 of the power module 92. Accordingly, the use of the auxiliary control circuit 923 complicates the circuit of the power module 92 and increases the manufacturing cost thereof. Also, no signal is transmitted from the at least one power module 92 for notifying the control module 91 of the information about the compensation capacity of the power module 92 or the number of power modules 92 when more than one power module 92 is used. As a result, it still causes much inconvenience to a manufacturer in assembly, test, manufacture, and service due to a requirement for manually setting the above information when the power module 92 is first coupled with the control module 91.
The present invention provides a modularized active power filter including a control module and at least one power module. The control module comprises a module counting circuit while the power module comprises a module state sensing circuit. The module counting circuit receives a signal from the module state sensing circuit, such that the control module is able to identify the number of applied power modules automatically. Thereby, the control module generates one set of control signals to at least one power module corresponding to the number thereof, such that a final compensation capacity is provided by a single power module or jointly provided by plural power modules. Through the provided final compensation capacity, the harmonic currents of a load are filtered Out, and the reactive power is compensated. Furthermore, in comparison with the conventional modularized active power filter 9, the auxiliary control circuit 923 and the PWM circuit 924 thereof can be omitted since a PWM signal is transmitted from the control module to the parallel connected power modules directly in the present invention. Therefore, when the power module is damaged or the compensation capacity is changed, it is easy to process a replacement or change for the power module. Consequently, the flexibility of compensation capacity is improved, and the time for service is shortened. Moreover, the modularization of the active power filter is suitable for mass production, and the manufacturing cost is lowered.